1. Field of the Invention
This invention relates to a silicone crumb which is useful as a hydraulic material which can be subjected to temperatures. such as 600.degree. F. over long periods of time and maintain their usefulness.
2. Background Information
Material used in hydraulic systems are mainly fluid, i.e. gases or liquids. Such systems are used in machines to move parts of the equipment, for example in a hoist to lift automobiles. Other systems using hydraulics are the brakes on vehicles. The use of solid materials for hydraulic purposes is much more limited because most solids do not readily move and transfer pressure as conveniently as gases and liquids. Silicone fluids are used in hydraulic systems but these systems must be closed systems in the same manner as other hydraulic systems using gases or liquids. If the systems, using gases or liquids, are not closed, loss of material will occur and the effectiveness of the hydraulics is lost.
Other disadvantages of liquids is that if a leak should occur, it may cause undesirable environmental conditions and the cleanup may be difficult. Some liquids may be unstable under high temperature conditions and solidify or deteriorate such that the hydraulic system is ineffective for its purpose.
Solid materials which can be deformed might be suggested as useful material for hydraulic purposes and for use in methods for thermal expansion molding for composites. These thermal expansion molding methods use solid elastomeric materials in molds to cause pressure against a composite during the molding process. Such thermal expansion molding methods have the disadvantage that the determination of the pressure against the composite is difficult and requires very careful filling of the elastomeric mold portion because either under or over filling can cause unwanted pressures which result in bad composites. Because of the difficulty of using solid elastomeric materials in the thermal expansion molding methods, the expense is high enough to cause these methods to be used only in very special applications in which the expense would be acceptable. However, not much is reported for use of solid materials for hydraulic purposes because the solid materials do not flow.
Bruner in U.S. Pat. No. 3,843,601, issued Oct. 22. 1974, describes a hydraulic elastomer which is reported to have a high cross-link density and a high proportion of free chain ends. Bruner crumbles his elastomer under high shear stress to a powder which flows like a viscous fluid through a narrow orifice. Bruner reports that silicone elastomers are desirable materials for their high thermal stability, have high compressibility, and can be forced through an orifice. However, the silicone elastomers do not easily flow back into their original position because they are relatively hard. Oil has been used to plasticize the elastomer but this results in oil bleed which leaks out of the system. Bruner teaches that an improved hydraulic elastomer can be obtained by curing a linear vinyl containing siloxane copolymer which is made up of dimethylsiloxane units and methylvinylsiloxane units and which has a molecular weight between 20,000 and 200,000 corresponding to viscosities between 1000 and 1,000,000 centipoise at 25.degree. C. The vinyl is present in Bruner s copolymer in an amount of from 0.1 to 0.9 mole percent. Bruner teaches that even with the optimum vinyl content it is found that the ultimate properties of the cured elastomers are not as good as when the viscosity of the fluid is at least 5000 cp and that good physical properties are obtained when the viscosity of the fluid reaches 1,000.000 cp. Bruner cures his vinyl containing copolymer with peroxide. Bruner teaches that those cured elastomers of his which have durometers on the Shore A scale of 8, 10, 19, 22, and 26 are outside the acceptable range of hardness which means that the acceptable hardness are those which have durometers on the Shore A scale between 11 and 18.
The solid materials which are useful for expansion molding of composites will require utility at temperatures above 500.degree. F. if some of the new curable materials are used in making the composites, because many of these new curable materials require cure temperatures above 500.degree. F. such as 600.degree. F. or higher. Silicone crumbs are described by Beck et al. in U.S. Pat. application Ser. No. 835,496, filed Mar. 3, 1986, now U.S. Pat. No. 4,686,271, and assigned to the same assignee as the present application and is hereby incorporated by reference to show hydraulic silicone crumbs. These silicone crumbs of Beck et al are useful materials in expansion molding methods but when heated to higher temperature, such as 600.degree. F. the silicone crumb begins to loose its effectiveness as a hydraulic material. This loss of hydraulic effectiveness is believed to be the result of decomposition products causing the crumb particles to stick together and thus the crumb cannot be readily moved, such as through a pipe. An improved hydraulic silicone crumb is therefore desirable, namely one in which the decomposition is minimized or eliminated at the higher temperatures, such as above 500.degree. F.
The use of a heat stability additive in silicone rubber is known for example iron oxides and iron compounds have been known for a long time as heat stability additives to silicone rubber. Other heat stability additives which are known, are the cerium compounds and metallic zirconates. Kishimoto et al in U.S. Pat. No. 4,070,343, issued Jan. 24. 1978. teach, as a heat stability additive to organosiloxane polymers, a reaction product of an alkali metal siloxanolate having at least three organosiloxane units per molecule with a cerium salt of an organic carboxylic acid or cerium chloride and an organic carboxylic acid salt or alkoxy compound of zirconium, titanium. or iron. These additives taught by Kishimoto et al can be used to improve the heat stability of silicone sealants, silicone rubber gaskets, silicone fluids for hydraulic systems such as in automobile brake systems, and many other areas wherein silicone fluids and rubbers are used. The amounts of weight loss shown by Kishimoto et al of 2.6 to 6.7 weight percent after heating at 250.degree. C. for 48 hours would be unacceptable for crumb rubber applications used exposed to temperatures above 500.degree. F.
A heat stability additive for making endblocked methylpolysiloxane fluids more resistant to thermally induced siloxane rearrangement is described by Halm in U.S. Pat. No. 4,122,109. issued Oct. 24. 1978. which is hereby incorporated by reference to show the preparation of the heat stability additives used in the silicone crumb of the present invention and its preparation, Halm teaches, as the heat stability additive, the product obtained by mixing endblocked methylpolysiloxane fluid with an organometallic compound of zirconium, titanium, or hafnium. and heating the mixture to cause the decomposition of the organometallic compound; or an organometallic siloxy compound of zirconium, titanium, or hafnium with at least one Si-O-Zr, Si-O-Ti, and Si-O-Hf bond mixed with endblocked methylpolysiloxane. Halm also teaches that an improved heat stability additive can be formed by adding an organosilicon hydride compound. However, Halm reports teaches that organosilicon hydride compounds should not be added to a metal-containing methylpolysiloxane which contains more than 0.1 weight percent metal, because the loss of silicon-bonded hydrogen and the release of hydrogen gas may occur, particularly in the presence of moisture. Halm teaches that the amount of additive for endblocked methylpolysiloxane fluids is an amount sufficient to provide from 0.001 to 0.1 parts by weight of metal per 100 parts by weight of fluid to improve the resistance to thermal siloxane rearrangement.